This invention relates generally to objective lenses, and, more particularly, to objective lenses used in the recording and playback of signals recorded optically on rotatable discs.
It is well known that information may be recorded on an information track on a disc by means of a modulated laser beam. The information track takes the form of a series of pits in the disc surface. When a playback laser beam is directed onto the surface, the information is retrieved from the information track, since the playback beam is selectively reflected by the presence or absence of pits in the surface. This technique is generally well known in the video recording field, and may also be used for the storage and retrieval of digital information, such as may be used in conjunction with a computer system.
The information is recorded on the disc either in a spiral track or in a plurality of concentric circular tracks on the disc surface. In either case, means must be provided for focusing the laser beam onto the disc surface in a spot size of the order of one micron in diameter. For recording purposes, the focused laser beam must have sufficient power to effect the necessary deformation of the surface, typically by means of a controlled combustion or melting process. Accordingly, a relatively high numerical aperture objective lens is desirable for this application. Moreover, the working distance or clearance between the lens and the recording surface needs to be great enough to minimize the possibility of inadvertent contact with the disc surface.
Although it is possible to design an objective lens with the desirable characteristics of high numerical aperture and relatively large working distance, such a lens would typically be bulky and massive. Unfortunately, another requirement for an objective lens used in the recording of information on a disc is that the total mass of the lens be minimized, to facilitate rapid movement of the lens both in a radial direction, to move from one information track to another and to automatically align with a given information track, and in a direction perpendicular to the disc surface, to ensure automatic focusing of the beam.
Another difficulty is that lasers typically used for recording in such systems, such as argon-ion lasers, provide coherent light at more than one wavelength. Although the laser beam could be filtered to provide a monochromatic source, this would substantially reduce the total power of the beam and would therefore necessitate a laser of substantially higher power for recording purposes. Accordingly, the objective lens used to focus the beam on the disc surface should ideally be color corrected to provide a properly focused beam over a range of frequencies including those generated by the laser.
It will be appreciated from the foregoing that there has been a significant need for an objective lens having the desirable characteristics of a relatively large numerical aperture, a relatively large working distance, and low total mass, and which is color corrected over the range of wavelengths produced by the laser. The present invention satisfies this need.